1. Field of the Invention
This invention relates generally to a communication system, and, more particularly, to a wireless communication system.
2. Description of the Related Art
Mobile units communicate with a conventional wireless communication network over wireless communication links (or air interfaces) with one or more base stations or nodes-B. In one exemplary network architecture, the base stations are communicatively coupled to a controller, which is connected to the wireless communication network. For example, in a Universal Mobile Telecommunication System (UMTS) network, one or more base stations are communicatively coupled to a radio network controller (RNC) via an asynchronous transfer mode (ATM) switch. The base station typically handles layer-1 services such as channel coding and/or channel decoding. The radio network controller provides layer-2 services such as radio link control (RLC), packet data convergence protocol (PDCP), and medium access control (MAC), as well as layer-3 services such as terminating Internet Protocol traffic with the network.
A mobile unit in soft handover can maintain concurrent wireless communication links with more than one base station associated with a radio network controller. For example, a radio network controller may provide transport blocks to a plurality of base stations in an active set associated with a mobile unit. The base stations in the active set may then provide information in the transport blocks to the mobile unit over a corresponding plurality of air interfaces. The mobile unit combines information received over the plurality of air interfaces to decode the transport blocks. Similarly, the base station may provide information to the plurality of mobile units over the plurality of air interfaces. The radio network controller may then use the information received from the plurality of base stations to form packets that may be provided to the network. However, soft handover requires synchronization between backhaul links from the base stations to the radio network controller. The base stations and/or radio network controller may include jitter buffers to maintain the required degree of synchronization.
In one alternative wireless network architecture, base station routers (BSRs) may be used to provide wireless connectivity to mobile units. The base station routers are connected by a switch, which is communicatively coupled to the wireless communication network via a home agent. A base station router typically provides the functionality of a radio network controller and a base station in a single entity. For example, a base station router may provide layer-1, layer-2, and layer-3 services. Wireless communication networks that use base station routers do not require synchronous backhaul links. However, soft handover may be difficult to implement in wireless communication networks that include base station routers. For example, each base station router would need a relatively large jitter buffer to maintain synchronization between multiple base station routers. Large jitter buffers may lead to long round-trip delays.
Network architectures that include base station routers may therefore implement hard handovers (HHOs) between a source base station router (BSR) and a target BSR. The inter-BSR HHO is inspired by the standard inter-RNS HHO as described in 3 GPP TS 25.931. The inter-BSR HHO procedure may extend the inter-RNS handover procedure with support for lossless frame selector relocation. A conventional inter-RNS HHO procedure starts with a mobile unit reporting one or more channel conditions based on pilot strengths for the various pilots the mobile unit is able to decode to the source BSR. The source BSR determines whether a hard handover to a selected target BSR should be performed and, if so, the source BSR communicates with the target BSR to allocate uplink and downlink wireless channel resources. The target BSR may then initiate a new downlink radio channel. Once the downlink channel has been initiated, the target BSR responds to the source BSR, which transmits a layer-3 channel reconfiguration message over a signaling channel (DCCH-2) to the mobile unit. On reception, the mobile unit acquires radio frame synchronization before initiating a new uplink. As soon as the target BSR receives the request to start a new downlink channel, it also activates internal searchers to locate the new uplink from the mobile unit. Once the target BSR acquires uplink synchronization, it sends a Mobile Internet Protocol (MOIP) registration request to a home agent (HA) to inform the home agent of the new care-of address of the Mobile IP Foreign Agent (FA) inside the target BSR. On successful registration, the home agent tunnels downlink data to the target BSR.
The conventional inter-RNS hard handover procedure is not a lossless handover. First, data queued inside the radio layer control (RLC) bearers is lost during the handover, due to an implicit or explicit RLC reset. Accordingly, incomplete packets both in uplink and downlink may be discarded during the conventional hard handover. Since there is no mechanism available for layer-2 retransmission of the lost data, higher networking layers need to resolve the loss, e.g., using a Transmission Control Protocol (TCP) retransmission. Internet Protocol (IP) packets queued for transmission in the downlink may also be lost because the conventional inter-RNS handover does not typically forward this data between the source BSR and the target BSR. Consequently, data that is in flight from a home agent to the source BSR while the mobile unit is reconfiguring the uplink and downlink may also be lost because this data is not forwarded from the source BSR to the target BSR. The impact of not forwarding such data can be severe, as the MoIP re-registration does not happen until uplink synchronization has been re-established.